Poly(methyl methacrylate) bone cement, which has been used routinely for the fixation of metallic or plastic implants in joint replacement surgery for about thirty years, is based on a monomer which can be admixed with a curing system and applied to the bone under conditions which permit the monomer to polymerize in situ in the bone. However, as the polymerization of methyl methacrylate is an exothermic reaction, resulting in the evolution of heat, it is desirable that the amount of monomer in the unset bone cement be minimized in order to avoid any damage to the bone tissue. One technique for minimizing the monomer concentration in the unset bone cement, while still achieving the benefits of the characteristics of the resulting poly(methyl methacrylate), is to include a substantial amount of small particles of the polymer in the unset bone cement composition, e.g., about 2 grams of polymer powder per milliliter of monomer. The size of the polymer powder particles is generally in the range of about 30 to about 150 .mu.m in diameter. As the polymerization of methyl methacrylate is accompanied by a substantial decrease in volume, the reduction in the amount of the monomer required in the bone cement by the inclusion of a large amount of poly(methyl methacrylate) powder also minimizes the shrinkage experienced during the setting of the bone cement. Thus, methyl methacrylate bone cement is most commonly available in two standard packages, one containing 40 mg. of poly(methyl methacrylate) powder and the other containing 20 ml of liquid methyl methacrylate monomer.
In order to effect the polymerization of the monomer, the bone cement composition also contains an initiator and an activator. Thus, poly(methyl methacrylate) bone cement has generally been formed by operating room personnel admixing a powdery component, containing the polymer powder and an initiator, and a liquid component, containing the liquid monomer and an activator, to obtain the unset cement, which must then be quickly applied to the bone, as the cement composition sets within a few minutes. For example, poly(methyl methacrylate) bone cement can be prepared by mixing together the following ingredients: liquid methyl methacrylate, powder particles of poly(methyl methacrylate), benzoyl peroxide powder as the polymerization initiator, and N,N-dimethyl-p-toluidine (DMPT) as an activator. Occasionally, a poly(methyl acrylate) or a polystyrene copolymer has also been included in the polymeric components for forming the bone cement.
As early as 1975 the importance of reducing the porosity of the set bone cement in order to improve the mechanical properties of the set bone cement was recognized. This porosity is a source of mechanical weakness of the cement material and can cause early failure of fixation of the implant. The porosity of set bone cement can be measured either as a volume percentage or as a percentage of cross-sectional area occupied by voids. Regardless of the method of measurement, the generally reported values for set bone cement, resulting from hand-mixed bone cement of regular viscosity, are in the range of about 5 to about 16 percent.
The porosity of set bone cement can be the result of the entrapment of air during the mixing and transferring process, the presence of air spaces between polymer particles at the time of the addition of the other components to the polymer particles, the generation of voids in the unset bone cement as a result of evaporation or boiling of the monomer during the mixing process or during the setting of the bone cement, the thermal expansion of existing bubbles, and the presence of cavitation voids. Of these causes, the most common source of porosity, as well as the most easily controlled, is air entrapment during the mixing and transferring process.
It is well known that vigorous hand mixing of bone cement components, under atmospheric conditions, increases the amount of air in the resulting bone cement mixture. Because of the viscous nature of the resulting bone cement mixture, only large bubbles, e.g., having a diameter greater than 1 cm, can easily migrate to the surface of the unset bone cement mixture, leaving a substantial number of voids in the bone cement mixture with a diameter of less than 1 cm.
When the powdery component and the liquid monomeric component are mixed, a doughy mixture is formed, and the initiator and activator in the mixture start polymerization of the liquid monomeric component. When mechanical agitation is employed during the mixing of the liquid and powder components, air can be trapped within the resulting doughy mixture. The entrapped air causes porosity in the resulting solidified cement mixture.
While porosity generally occurs throughout the body of polymerized bone cement as a result of the above listed causes, there are also regional variations, particularly in the cement mantle surrounding a femoral hip stem. Greater porosity is noted in the proximal cement mantle. In addition, the porosity is preferentially concentrated at the cement-prosthesis interface of a cemented femoral stem in a total hip replacement due to the rheological behavior of the bone cement during the implant insertion. The concentration of the porosity at the interface greatly exceeds the average porosity of the bulk cement. This interfacial concentration of porosity cannot be reduced by the prior centrifugation of the bone cement.
Considerable effort has been directed to reducing the amount of porosity in set bone cement, with centrifugation, vacuum mixing, and ultrasonic agitation of the unset bone cement mixture being the most frequently employed techniques. Currently, centrifugation and vacuum mixing are being used clinically. As an example, the prepackaging of the powdery component of the bone cement under high vacuum in a mixing vessel, which permits the addition of the liquid monomer component to the powder without the introduction of air into the vessel, thereby minimizing the presence of air bubbles in the resulting cement, is described in Chan, U.S. Pat. No. 4,973,168, and Chan, U.S. Pat. No. 5,100,241. However, the use by operating room personnel of either high vacuum mixing or centrifugation in the operating room can significantly complicate the procedure.
Storing liquid bone cement monomer in a non-glass container presents a major engineering and manufacturing challenge. The problem is even greater where the container has a sliding piston, as it is extremely difficult to achieve a hermetic seal with a sliding piston in the presence of liquid bone cement monomer.
Accordingly, it is desirable that a technique be found to provide a poly(alkyl methacrylate) bone cement kit which can be readily utilized in the operating room to prepare the poly(alkyl methacrylate) bone cement composition from only two prepackaged components without requiring either high vacuum mixing or centrifugation.